1. Field of the Invention
This invention relates to a circuit for compensating for waveform shaping errors, and more particularly to such a compensating circuit for controlling the speed of a capstan motor by using an FG signal.
2. Description of the Related Art
A capstan servo circuit is currently known which controls the speed of a capstan motor such as that of a VTR. In the servo circuit, a frequency generator mounted on the shaft of the capstan motor outputs an FG signal according to the r.p.m. of the motor. This FG signal is wave shaped and is multiplied for use as a servo signal.
FIG. 1 of the accompanying drawings is a block diagram showing such a conventional capstan servo circuit. An FG signal from a frequency generator mounted on the shaft of a non-illustrated capstan motor is inputted to an amplifier 10 and is amplified thereby. The amplified FG signal is inputted to the inverting input terminal of a comparator 12a and the inverting input terminal of a comparator 12b and is compared with their respective predetermined reference levels. The output of the comparator 12a is inputted to the set terminal S of an RS flip-flop 14, while the output of the comparator 12b is inputted to the reset terminal R of the RS flip-flop 14. As a result, the output of the RS flip-flop 14 will be a rectangular signal having a pulse width exceeding the predetermined reference levels of the comparators 12a, 12b. The amplifier 10, the comparators 12a, 12b and the RS flip-flop 14 constitute a waveform shaping unit 1.
The FG signal shaped into a rectangular waveform is multiplied to a double frequency by a frequency multiplier 16. An FV counter 18 counts first the time t1 from a rise to a subsequent fall of the waveform-shaped FG signal and then the time t2 from a fall to a subsequent rise of the waveform-shaped FG signal. A pulse width modulator 20 modulates a pulse width of the resulting FG signal by the count values and outputs the modulated signal as a servo signal for the capstan motor.
In this conventional capstan servo circuit, waveform shaping errors are reduced by increasing the gain of the amplifier 10, and any increase of the waveform shaping errors is prevented by narrowing the comparison levels of the comparators 12a, 12b.
However, with this arrangement, waveform shaping errors would depend on the varying characteristics such as offset or distortion factor of the amplifier and comparators so that uniform characteristics are difficult to achieve.